A fuel cell system has been put into practical use. This system includes a sealed fuel gas supply space on one surface side of a power generation layer member and an oxygen supply layer on the other surface side of the power generation layer member. The power generation layer member takes in hydrogen ions from the fuel gas supply space and allows the hydrogen ions to react with oxygen on a surface on the oxygen supply layer side, thereby generating power. The oxygen supply layer is not only a supply path for providing a required amount of oxygen to the surface of the power generation layer member, but is also a diffusion (or forceful discharge) path for transporting out water molecules generated in the power generation layer member.
U.S. Pat. No. 6,423,437 shows a fuel cell system in which fuel cells, each having a power generation layer member, are stacked and connected in series. Oxygen in the atmosphere is taken in through an opening on a side surface of each fuel cell. Water in the oxygen supply layer is evaporated and diffuses to the atmosphere through the same opening. As the power generation layer member, a membrane electrode assembly, in which a porous conductive catalyst layer is formed on both surfaces of a polymer electrolyte film, is adopted. A side surface bordering the opening in a plate-shaped oxygen supply layer having three-dimensional air permeability is opened to the atmosphere. Oxygen taken in from the side surface of the oxygen supply layer diffuses three-dimensionally in the oxygen supply layer and is supplied to the entire surface of the membrane electrode assembly through one bottom surface of the oxygen supply layer. Water molecules generated in the membrane electrode assembly are taken in the oxygen supply layer as water vapor, moved to the side surface in accordance with the concentration gradient of the water vapor, and diffused into the atmosphere through the opening.
Japanese Patent Application Laid-Open No. 2005-174607 shows a fuel cell system, which forcefully sends the atmosphere from one side surface to the other side surface of an oxygen supply layer to allow it to flow. Herein, a separator with a groove-shaped air flow path passing through the opposed side surfaces of the fuel cell system is stacked on the oxygen supply layer. Then, the tissue density of the oxygen supply layer in contact with the air flow path is changed in the thickness direction. The tissue density of a surface layer in contact with the air flow path and a surface layer being in contact with the membrane electrode assembly is set to be higher than that of an intermediate layer, whereby the water-retaining property of the intermediate layer is enhanced.
Japanese Patent Application Laid-Open No. 2002-110182 shows a fuel cell system in which a catalyst layer is formed on a surface on a polymer electrolyte film side of an oxygen diffusion layer stacked on a power generation layer member. The supply of oxygen and the discharge of water vapor in the oxygen diffusion layer are performed passively by natural diffusion. The oxygen diffusion layer is allowed to pass through in the thickness direction to form an infinite number of through-holes with an aperture of 100 μm or less at a density of 400 holes per mm2, whereby the diffusion performance in the thickness direction is enhanced. Each through-hole (in a cone shape), the cross-sectional area of which increases from a polymer electrolyte film side to a surface on the opposite side, increases the contact area on the polymer electrolyte film side and increases the strength of the oxygen diffusion layer, while decreasing the passage resistance of oxygen and water vapor.
Japanese Patent Application Laid-Open No. 2005-353605 discloses a fuel cell system including a water-absorbing material at an oxygen electrode, which sucks out water using capillary action, thereby suppressing flooding.
It is desirable that an integrated fuel cell system used integrally with electronic equipment supply oxygen and discharge water vapor through the oxygen supply layer passively by natural diffusion. It is desirable for such a fuel cell system not to require a supply of power from the outside for activation, because a circulation mechanism and an atmospheric blower increase the number of parts, which is contrary to the desired miniaturization and reduction in weight of the fuel cell system. A fuel cell system shown by Japanese Patent Application Laid-Open No. 2005-174607 is predicated upon such a circulation mechanism and atmospheric blower.
However, in the case where the supply of oxygen and the discharge of water vapor in the oxygen supply layer are performed fully by natural diffusion, oxygen and water vapor move in opposite directions. Therefore, if the output current of the fuel cell system increases to increase the discharge amount of water vapor, there is a possibility that the supply of oxygen may be prevented. Particularly, in the case where fuel cells are stacked and water vapor is discharged through an opening on a side surface of each fuel cell, oxygen is hindered by the flow of water vapor directed to the opening, with the result that the oxygen is unlikely to reach a portion away from the opening.
When the supply of oxygen to the power generation layer member is hindered, the electromotive power decreases to reduce the power generation efficiency of the fuel cell. When the heat generation amount increases to cause a further increase in temperature as a result of the reduction in the power generation efficiency, the water vapor partial pressure in the oxygen supply layer increases and the oxygen partial pressure decreases, with the result that the supply of oxygen with respect to the power generation layer member is further hindered.
Also, when the water vapor partial pressure of the oxygen supply layer increases, the evaporation of water generated at the interface of the power generation layer member is hindered. Liquid water accumulates and locally floods the interface. In the flooded portion, the supply of oxygen ceases and power generation stops. Therefore, the current density in a portion that is not flooded increases, and the electromotive force of the fuel cell decreases. Then, when the operation is continued as it is, flooding spreads to a region where the current density has increased, leading to the flooding of the entire surface of the power generation layer member, which may result in the overall suspension of the power generation of the fuel cell.
Thus, compared with the active type system in which the atmosphere is forcefully circulated to the oxygen supply layer to forcefully discharge water vapor, in the passive type system, depending upon natural diffusion, it is necessary to set a current per unit surface area of the power generation layer member to be extremely small. When the current per unit surface area is extremely small, the area of the power generation layer member increases to enlarge a power generation portion, which may make this fuel cell system even larger than that the active type system.
A fuel cell system shown by Japanese Patent Application Laid-Open No. 2005-174607 sets the density of a surface layer of an oxygen supply layer in contact with a power generation layer member higher than that of an intermediate layer, thereby sucking up liquid water at an interface of the power generation layer member to the intermediate layer efficiently to vaporize and diffuse the liquid water. However, the water vapor supplied to the intermediate layer accumulates in the intermediate layer to hinder the diffusion of oxygen and the supply of oxygen to the power generation layer member through the intermediate layer until the water vapor is discharged through a surface layer on an opposite side where the density has increased. Then, the surface layer for actively accumulating water in the intermediate layer member increases the water vapor pressure in the intermediate layer, thereby making it difficult for oxygen to reach the power generation layer member.
A fuel cell system shown by Japanese Patent Application Laid-Open No. 2002-110182 is predicated upon the passive type system depending upon natural diffusion, thereby enhancing the water discharge performance from a power generation layer member to an oxygen supply layer. However, the water taken in the oxygen supply layer still moves in a direction opposite to that of oxygen in the oxygen supply layer due to natural diffusion of water vapor. That is, the water vapor partial pressure of the oxygen supply layer is not decreased so as to facilitate the evaporation of water generated in the power generation layer member, and the movement/diffusion of oxygen through the oxygen supply layer is not facilitated.
A fuel cell system shown by Japanese Patent Application Laid-Open No. 2005-353605 has a configuration in which a catalyst is surrounded by a water-absorbing material. This reduces the size of the catalyst portion, which makes it difficult for the fuel cell to exhibit sufficient performance.